Lithography is a critical step in the manufacturing of electronic integrated circuits. In the lithographic step, circuit design patterns first created on a mask are imaged on a silicon wafer using lithographic exposure tools (like everyday cameras), commonly known as optical exposure steppers. A circuit design pattern of a mask is imaged serially in a stepwise fashion on a plurality of locations in a regular array on the silicon wafer. Integrated circuits are manufactured by overlaying several circuit design patterns onto the silicon wafer locations using the exposure tools. As future integrated circuits require ever decreasing line widths, 1-X proximity X-ray lithography, a type of lithography in which the pattern on the mask is the same size as the image on the silicon wafer, becomes an increasingly important method to produce these circuits, because it uses electromagnetic radiation sources with much smaller wavelengths.
One main function of the lithography process, besides imaging the circuit pattern, is to align a subsequent circuit design pattern with previous levels of design patterns on the silicon wafer without introducing significant overlay errors. Excessive overlay error can prevent manufactured circuits from functioning. One of the sources of overlay error is a relative magnification mismatch between the mask and the wafer. The magnification mismatch is caused by heat associated with the process, which causes the silicon wafer and, therefore, the circuit patterns already formed in the silicon wafer, to expand by varying amounts at different times. As a result, subsequent mask patterns must be magnified to be the same size as the patterns already formed on the silicon wafer, and thereby minimize overlay error.
Unlike known optical exposure steppers, 1-X proximity X-ray exposure steppers (called X-ray steppers or X-ray aligners) do not have optical reduction lenses. An X-ray mask is held in proximity to the silicon wafer, typically 5 to 50 microns away from the wafer surface. The circuit design pattern on the mask resides on a thin membrane which allows transmission of X rays. Absorbers on some areas of the membrane prevent sufficient X rays striking those areas from reaching the wafer surface, which is coated with a layer of photo-sensitive film. Thus, the circuit pattern is imaged onto the wafer by the X rays passing through areas in the membrane on which there are no absorbers. Since the X-ray stepper does not contain optical lenses to match the pattern on the mask to the patterns on the silicon wafer, any magnification adjustment to minimize the magnification mismatch between the mask and the previous patterns on the wafer is extremely difficult.
Techniques of magnification correction have been proposed previously. U.S. Pat. No. 4,964,145 discloses using a piezoelectric film on the mask to create dimensional changes of the pattern. U.S. Pat. No. 5,155,749 discloses using a thermally controlled metal ring imbedded in an X-ray mask to enlarge or contract the mask pattern. Both of these techniques require additional process steps in the fabrication of the X-ray mask, thereby complicating the mask making procedures. Furthermore, a thermal source close to the exposure area can create other overlay errors, and such errors cannot be corrected.
Magnification without overlay error is complicated by the much greater stiffness of the mask member outside the membrane than in the membrane. This fact and the rectangular shape of the membrane in a circular ring makes pure magnification by the application of mechanical force so difficult. The areas of the mask membrane around the corners of the rectangular membrane produce higher stiffness which causes deformation of the membrane around the corners to be different from deformation of the membrane along its edges, between the corners.